The present invention relates to a focus control device that is provided in an image pickup apparatus, such as a video camera, and that controls a focus status of an image acquired by a lens portion and to a focus control method for the focus control device.
Nowadays, most of image pickup apparatuses, such as portable consumer video cameras and still cameras, have an automatic focus function. As a method for such automatic focus control, a contrast method using contrast information of an image signal acquired from image pickup light by a lens portion is widely known. In addition, as the contrast method, a hill-climbing method is described, for example, in Japanese Patent No. 2966458.
For example, such an automatic focus function always monitors whether it is in an in-focus state or not. If it is determined that it enters a not-in-focus state, an automatic focus operation is immediately performed. Accordingly, recovery to the in-focus state can be achieved as quickly as possible.
Generally, portable video cameras, which are assumed to be used by general end users, are used such that the powers of the portable video cameras are turned off at a time other than a time when photographing (recording) is actually performed. For example, when a user is carrying a portable video camera for an event and is not performing photographing, the power of the portable video camera is turned off. In other words, portable video cameras are not used to continuously perform photographing over a long time or over a long period.
As apparatus systems using video cameras, monitoring camera systems are also available. For example, a video camera used for such monitoring camera systems is installed in a fixed position and is continuously operated over a long period.
In the existing situation, in many cases, an automatic focus function provided in a portable video camera apparatus is also provided as an automatic focus function in a monitoring camera. This means that such a monitoring camera also starts an automatic focus operation immediately in response to a change to a not-in-focus state, as described above.
A change from an in-focus state to a not-in-focus state may be detected not only when a human being or an object enters an image pickup area but also when the brightness of a picked-up image changes.
As described above, a monitoring camera continuously operates over a long period. Thus, although it depends on the environment in which the monitoring camera is installed, the frequency of the automatic focus operation performed by the monitoring camera during a certain period is significantly greater than the frequency of the automatic focus operation performed by a portable video camera.
In order to achieve an in-focus state using the automatic focus function, a focus lens in a lens unit is moved along an optical axis direction. In order to move the focus lens, a physical driving mechanism including a motor, a gear, and the like is provided. Thus, performing an automatic focus operation with high frequency means performing an operation of the above-mentioned driving mechanism with high frequency. This causes further wear of components forming the driving mechanism, thus resulting in a reduction in durability of the monitoring camera apparatus.